Support create_share_from_snapshot_support extra spec

The snapshot_support extra spec has always meant two things: a driver can take snapshots and create shares from snapshots. As we add alternate snapshot semantics, it is likely that some drivers will want to support snapshots and some of the new semantics while being unable to create new shares from snapshots.

This work adds support to manila client for the new extra spec in manila server, create_share_from_snapshot_support.